1. Field of the Invention
The present invention relates to an electric double layer capacitor comprising electrode terminals which are attached to a metallic casing in an electrically insulated state. In particular, it relates to an electric double layer capacitor wherein the rotation of the electrode terminals along with a locking element is prevented.
2. Discussion of Background
Recently, an electrochemical element such as an electric double layer capacitor and so on having a large capacity and a high output, which is applicable to a vehicle-mounted driving power source or the like, has been noted and developed.
As a structure for a large-sized electric double layer capacitor suitable for discharging a heavy current, an electric double layer capacitor having a rectangular structure is known as described in Japanese Unexamined Patent Publications: JP-A-4-154106, JP-A-7-264715, JP-A-7-307250, JP-A-7-272986, JP-A-8-45793 and so on. Such type of electric double layer capacitor is shown in FIG. 1. Namely, a plurality of positive electrodes 25 each comprising a generally rectangular metallic current collector on both sides of which electrode layers are formed and each provided with a current collecting lead 52, and a plurality of negative electrodes 25' having the same structure as the positive electrodes and provided with current collecting leads 54 are alternately stacked by interposing therebetween separators to form a stacked assembly 50 from which the current collecting leads 52, 54 are extended. The positive current collecting leads 52 are electrically connected by means of caulking or welding to a positive current collecting terminal 75 attached to a cover member 65. In the same manner as above, the negative current collecting leads 54 are electrically connected to a negative current collecting terminal 77 attached to the cover member 65. The stacked assembly 50 is received in a bottomed rectangular casing 60 filled with an electrolyte. Then, the cover member is hermetically fitted to the rectangular casing.
There has been known an electric double layer capacitor having a cylindrical structure as disclosed in Japanese Unexamined Patent Publications: JP-A-154106, JP-A-7-307250, JP-A-8-45795 and so on. Such type of electric double layer capacitor is fabricated as follows. A belt-like positive electrode obtained by forming electrode layers composed mainly of an activated carbon or the like on both surfaces of a metallic current collector and a belt-like negative electrode having the same structure as the positive electrode are stacked by interposing a belt-like separator. The stacked product is rolled to form a rolled assembly. The rolled assembly is received in a bottomed cylindrical metallic casing filled with an electrolyte. Current collecting leads drawn from the positive electrode and the negative electrode are respectively connected to electrode terminals attached to an electrically insulating cover member, and then, the cover member is fitted to the cylindrical metallic casing.
Since the electric double layer capacitor used for a vehicle-mounted driving power source is required to withstand vibrations and impacts occurring during the driving of a vehicle and to have a mechanical strength free from deformation while providing a high hermetic condition in the same manner as a vehicle-mounted battery, it is desirable that the casing and the cover member are made of a metallic material. In this case, the electrode terminals must be completely electrically insulated from the metallic cover member. Accordingly, it is necessary for an insulating material to have resistance to the electrolyte. Also, a sufficient care should be taken of a hermetically sealing property of the capacitor.
The inventors of this application tried to use a washer of synthetic resin and a ring of synthetic elastomer having a solvent resistance and hermetic properties to obtain a sufficient insulation to the electrode terminals and the metallic members. However, they have found that it is difficult to obtain a successful achievement. Namely, they have found a phenomenon that when screw terminals for the electrode terminals are to be fixed by means of nuts, the screw terminals are easily rotated along with the turning of the nuts for fastening.
The rotation of the screw terminals along with the nuts makes it impossible to completely fasten the washer of synthetic resin and the ring of synthetic elastomer whereby it is difficult to assure a sufficient insulation and hermetic properties.
If a capacitor having such a defect is used for a vehicle-mounted driving power source or the like, the screw terminals are loosened and rotate due to vibrations and impacts received. Then, the overall electrode terminals are rotated along with the fastening means with a result that there is a danger that the current collecting leads fixed to the current collecting terminals of the electrode terminals by caulking or the like may be cut off from the terminals due to a pulling force or a twisting force applied to the electrode terminals. If the screw terminals are loosened due to vibrations or the like, it is necessary to fasten the screw terminals again. However, the fastening operation may cause the twisting of the current collecting leads connected to the electrodes in an opposite direction.
Japanese Unexamined Utility Model Publication JP-U-52-67944 discloses such a technique that tinplate is used as an anti-turning piece for preventing the rotating of the screw terminal; the tinplate is bonded to the nut for the screw terminal by soldering, and the tinplate is bent to wrap a pressing plate. However, such a technique has disadvantages that impurities in a soldering material may enter into the capacitor; the anti-turning mechanism occupies a relatively large volume in the capacitor, and the capacity of the capacity per unit volume is small.